Environmental Engineering Reference
In-Depth Information
therefore subject to the ef ects of perturbations. A drought that eliminates
key species in a complex ecosystem, for example, will have widespread
repercussions on the animals that depend on them.
More recent work (Tilman, 1996) has shown that there exists an impor-
tant distinction between the properties of a community and its individual
species, so although diverse ecosystems are more stable than less diverse
ecosystems the populations within them can have great variability. From
this perspective, what matters is the stability of the community or eco-
system, not their individual populations. There is some experimental
evidence to support these assertions. Tilman and Downing (1994) have
shown that an ecosystem with many species is more likely to be stable even
though the populations of individual species may experience considerable
l uctuations.
Resilience is a further factor that refers to properties of the stability of
a system. The traditional concept of resilience is a measure of the speed of
return to an equilibrium state after an ecosystem has been disturbed (Pimm,
1984; O'Neill
et al., 1986). Alternative dei nitions have been proposed by
Holling (1973). He describes resilience as the propensity of an ecosystem to
retain its functional and organizational structure following a disturbance.
Expressed another way, resilience is the amount of disturbance that can
be absorbed before the system changes its structure by changing the vari-
ables that control how the system behaves (Holling, 1973). A characteristic
feature of 'Holling-resilience' then is that though the system parameters
(net primary production, or system growth rates, species composition)
may change after disturbance, a resilient community will return quickly
to equilibrium after disturbance is removed. A resilient ecosystem does
not necessarily imply that all of its component populations are stable.
Environmental perturbation may result in the extinction of an individual
species without af ecting ecosystem function or resilience. Holling (1973)
distinguishes between stability as a property associated with individual
populations of an ecosystem, and resilience as a property of an ecosystem.
Early work by Holling (1973) has suggested that, in general, the resili-
ence of an ecosystem is an increasing function of the diversity of that
system. There is some empirical evidence to support this view. In a series
of i eld experiments in drought-af ected grasslands in Minnesota, Tilman
(1996) has shown that species-poor plots were less productive in terms of
biomass than species-rich plots (see Figure 4.1A). He also demonstrated
that species-poor plots were more greatly harmed by drought (less resist-
ant), took longer to return to pre-drought conditions (less resilient) and
were less stable than species-rich plots. Tilman et al. (1997) also demon-
strated that plots with lower functional diversity had lower productivity in
biomass terms than plots with high functional diversity (see Figure 4.1B).
